A known cap of the kind described above includes a grip housing on which a sealing ring is supported. The grip housing includes a threaded stub which threadably engages a thread of a tank opening when rotated about the central axis of the grip housing whereby the seal is pressed against an end face of the tank opening so that a tight closure is provided.
A ventilating arrangement is provided in order to equalize the pressure in the tank and is mounted in the grip housing so as to be coaxial with respect to the central axis thereof. With this arrangement, the opening of the ventilating arrangement is almost always substantially horizontal on the upper surface of the grip housing so that there is a danger that the ventilating arrangement will become blocked by chips or other dirt particles which can become lodged at the region of the opening on the upper surface.